Association between Hashimoto thyroiditis and clinical outcomes of papillary thyroid carcinoma: A meta-analysis

Objective To assess association between Hashimoto thyroiditis (HT) and clinical outcomes of papillary thyroid carcinoma (PTC). Methods Databases including Pubmed, Embase, Cochrane Library, and Web of Science were searched. Weighed mean differences (WMDs) and odds ratios (ORs) were used to evaluate association between HT and clinical outcomes of PTC, and the effect size was represented by 95% confidence intervals (CIs). Heterogeneity test was performed for each indicator. If the heterogeneity statistic I2≥50%, random-effects model analysis was carried out, otherwise, fixed-effect model analysis was performed. Sensitivity analysis was performed for all outcomes, and publication bias was tested by Begg’s test. Results Totally 47,237 patients in 65 articles were enrolled in this study, of which 12909 patients with HT and 34328 patients without HT. Our result indicated that PTC patients with HT tended to have lower risks of lymph node metastasis (OR: 0.787, 95%CI: 0.686–0.903, P = 0.001), distant metastasis (OR: 0.435, 95%CI: 0.279–0.676, P<0.001), extrathyroidal extension (OR: 0.745, 95%CI: 0.657–0.845, P<0.001), recurrence (OR: 0.627, 95%CI: 0.483–0.813, P<0.001), vascular invasion (OR: 0.718, 95%CI: 0.572–0.901, P = 0.004), and a better 20-year survival rate (OR: 1.396, 95%CI: 1.109–1.758, P = 0.005) while had higher risks of multifocality (OR: 1.245, 95%CI: 1.132–1.368, P<0.001), perineural infiltration (OR: 1.922, 95%CI: 1.195–3.093, P = 0.007), and bilaterality (OR: 1.394, 95%CI: 1.118–1.739, P = 0.003). Conclusions PTC patients with HT may have favorable clinicopathologic characteristics, compared to PTCs without HT. More prospective studies are needed to further elucidate this relationship.

Background Hashimoto thyroiditis (HT) is a chronic inflammation of the thyroid gland initially described over a century ago, which is now considered the most common autoimmune disease [1,2]. An incidence is estimated to range from 0.3 to 1.5 cases per 1,000 people, with a prevalence of 5-10% in the overall population [3]. HT is characterized by hypothyroidism, the presence of serum antithyroglobulin and antiperoxidase antibodies, and widespread lymphocytic infiltration with depletion of follicular cells [4,5]. Thyroid cancer (TC) is the most common malignancy of the endocrine system, with papillary thyroid carcinoma (PTC) being the most prevalent form that accounts for 80% of all diagnosed TCs [6]. The incidence of PTC and HT is rapidly increasing in many countries [7,8]. The disease of PTC coexisted with HT presents an increasing trend year by year [9]. The coexistence of these two diseases has also been reported to range from 10% to 58% [10,11], which has aroused great concern.
The relationship between HT and PTC was investigated in several studies. Coexistent HT has been reported to be significantly associated with the less aggressive clinicopathologic characteristics of PTC [10,12]. Whereas several scholars observed HT is associated with a significantly increased risk of PTC [13]. Other studies have shown no connection between the presence of HT and PCT [14,15]. Moreover, the association with prognosis between HT and PC remains unclear. It is uncertain whether coexisting with HT in PTC represents a good prognosis or is simply the concurrence of both diseases. It is therefore reasonable to further evaluate the association between HT and PTC.
Herein, we conducted a meta-analysis with a multitude of outcome assessments included to explore the association between HT and PTC prognosis.
For each study, the following information was extracted, including author, year, country, study design, group, the number of patients, gender, age, subtype, tumor size, extent of surgery, tumor node metastasis stage, follow up, quality, outcomes.

Outcomes
The association between HT and clinical outcomes of PTC was assessed by lymph node metastasis (including lymph node metastasis, central lymph node metastasis, lateral lymph node metastasis), distant metastasis, extrathyroidal extension, recurrence, multifocality, invasion (includes vascular invasion, capsular invasion, perineural infiltration), bilaterality, number of deaths, AMES stage and MACIS score.

Statistical analysis
Software Stata (version 15.1, Stata Corporation, College Station, TX, USA) was used for statistical analysis. Weighed mean differences (WMDs) were statistics for measurement data, odds ratios (ORs) were used as effect indicators for continuous variables and frequency of events, and effect sizes were represented by 95% confidence intervals (CIs). A heterogeneity test was performed for each indicator. If THE heterogeneity statistic I 2 �50%, random-effects model analysis was carried out, otherwise, fixed-effects model analysis was performed. Each metaanalysis may create a false-positive or negative conclusion. Given this, TSA was conducted to reduce these statistical errors [16]. TSA is a methodology that combines an information size calculation (accumulated sample sizes of all included trials) to reduce type I error and type II error for a meta-analysis with the threshold of statistical significance (http://www.ctu.dk/tsa). TSA was used to quantify the statistical reliability of data in the cumulative meta-analyses by adjusting significance levels for sparse data and repetitive testing on accumulating data. Sensitivity analysis was performed for all outcomes, and publication bias was tested by Begg's test. Given the age imbalance between the case group and control group, an age-based sensitivity analysis was also applied (S2 File). P<0.05 was considered statistically significant.

Results
Initially, 1992 studies were searched according to the search strategy, and after duplicated removed, 1331 records were identified. With 174 full-text articles eligible for screening, 65 articles [5,8,9,12,13, were finally included in this meta-analysis, including 32 case-control studies, 27 cohort studies, and 6 cross-sectional studies. The flow chart depicting the study selection process is shown in Fig 1. Totally 47,237 patients were enrolled in this study, of which 12909 patients with HT and 34328 patients without HT. The characteristics of included studies are presented in Table 1.
Lateral lymph node metastasis. A total of 11 studies consisting of 1362 patients provided data to assess lateral lymph node metastasis. The heterogeneity test results were not statistically significant (I 2 = 43.3%), so the fixed-effect model was adopted. It was shown that HT was associated with a decreasing risk of lateral lymph node metastasis in PTC patients (OR: 0.845, 95% CI: 0.733-0.973, P = 0.02) ( Table 2, Fig 2C).

Distant metastasis
Distant metastasis was assessed in 11 studies comprising 151 patients. The fixed-effects model result showed that the HT group was at a lower risk of distant metastasis than the non-HT group (OR: 0.435, 95%CI: 0.279-0.676, P<0.001) ( Table 2, Fig 3).  Extrathyroidal extension. Totally 41 studies covering 13940 patients identified the association between HT and clinical outcome of PTC. The heterogeneity test results were statistically significant (I 2 = 74.1%), so the random-effect model was utilized. The result revealed that the risk of extrathyroidal extension in the HT group was lower than that in the non-HT group (OR: 0.745, 95%CI: 0.657-0.845, P<0.001) ( Table 2, Fig 4).
Recurrence. Sixteen studies containing 577 patients have assessed the recurrence. The result of fixed-effects model demonstrated that HT could decrease the risk of recurrence in PCT (OR: 0.627, 95%CI: 0.483-0.813, P<0.001) ( Table 2, Fig 5).
Multifocality. Multifocality referred to two or more foci found in the same lobe of the gland. A total of 44 studies embracing 10320 were included to evaluate multifocality. The heterogeneity test results were statistically significant (I 2 = 61.3%), so the random-effects model was used. The result illustrated that that the HT group had a higher risk of multifocality than the non-HT group (OR: 1.245, 95%CI: 1.132-1.368, P<0.001) (  Fig 6).
Capsular invasion. Nine studies including 2273 patients assessed the capsular invasion. No difference was found between the HT and non-HT groups in capsular invasion (OR: 1.234, 95%CI: 0.829-1.835, P = 0.300). Perineural infiltration. Two studies comprising 132 patients assessed the perineural infiltration.The perineural infiltration risk of the HT group was higher than that of the non-HT group (OR: 1.922, 95%CI: 1.195-3.093, P = 0.007) ( Table 2).

Bilaterality
Bilaterality referred to the presence of PTC in both thyroid lobes. Totally 18 studies involving 3421 were enrolled to assess bilaterality. Because the heterogeneity test results were statistically significant (I 2 = 78.9%), the random-effects model was adopted. The result showed that HT increased the risk of bilaterality in PTC patients (OR: 1.394, 95%CI: 1.118-1.739, P = 0.003) ( Table 2).

Deaths
Deaths. Death was identified in 6 studies containing 42 patients. There was no statistically significant in death between HT and non-HT groups (OR: 0.827, 95%CI: 0.386-1.773, P = 0.626).
Disease-specific death. Two studies including 82 patients were included to assess diseasespecific death. The result of fixed-effects model demonstrated that HT was not associated with disease-specific death in PTC (OR: 0.305, 95%CI: 0.059-1.585, P = 0.158).

AMES stage-low risk
A total of 4 studies embracing 1874 patients were enrolled to assess AMES stage-low risk. The heterogeneity test results showed that the differences were not statistically significant (I 2 = 0.0%), so the fixed-effects model was used for analysis. The low risk in the AMES stage represents a 20-year survival rate of 99%. The HT group had an advantage over the non-HT group in improving 20-year survival (OR: 1.396, 95%CI: 1.109-1.758, P = 0.005) ( Table 2, Fig 7).
MACIS score MACIS score. The higher the MACIS score, the worse the survival. Four studies involving 2733 patients were included to assess the MACIS score. The result uncovered that the the HT group had an advantage over the non-HT group in improving 20-year survival (WMD: -0.221, 95%CI: -0.306--0.137, P<0.001) (Table 2, Fig 8).

Publication bias
Begg's test was used for the assessment of publication bias. The result showed that there was no publication bias for lymph node metastasis (Z = 0.86, P = 0.  Table 2). However, there was a publication bias for bilaterality (Z = 2.20, P = 0.028) ( Table 2). The trim and fill method was applied to adjust data for  publication bias. The OR value of the random effects model before the trim and fill method was 1.394 (95%CI: 1.118-1.739). The random effects model was used to estimate the number of missing studies after 7 iterations, and the meta-analysis of all studies was conducted again. The OR value of the random-effects model after the trim and fill method was 2.858 (95%CI: 1.999-3.718), there was no significant change before and after the results, indicating that publication bias had little influence and the conclusions in the literature were relatively robust. TSA Lymph node metastasis. A total of 44 articles were included, with a total sample size of 28,813 cases. The required information size (RIS) was 34,021. The estimation of RIS was based  on the following variables: Type I error of 0.05, Type II error of 0.2, Power of 80%, Relative Risk Reduction of 20%, and Incidence in Control arm of 10%. The TSA results showed that the cumulative Z curve crossed the traditional boundary line and intersected the TSA boundary line, but did not reach the RIS line, indicating that although the expected sample size was not reached, the positive results were obtained in advance, which further verified that the HT group was better in the low risk of lymph node metastasis than the HT group.
Central lymph node metastasis. Seventeen articles with a total sample size of 15947 cases were included, the RIS was 61030 cases, and the RIS was estimated based on the following variables: Type I error of 0.05, Type II error of 0.2, Power of 80%, Relative Risk Reduction of 20%, Incidence in Control arm of 10%. The TSA results showed that the cumulative Z curve crossed the traditional boundary line, but did not reach the TSA boundary line and the RIS line, revealing that the expected sample size was not reached. In the future, more experiments are needed to verify the risk of central lymph node metastasis in the HT group versus the non-HT group.

Extrathyroidal extension
Forty-one articles were included, with a total sample size of 35,547 cases, and the RIS was 34,408 cases. It was shown that the cumulative Z curve crossed the traditional boundary line, but did not reach the TSA boundary line and the RIS line, indicating that the expected sample size was not reached. More trials are needed in the future to verify the reliability of the conclusion that the HT group has a lower risk of central lymph node metastasis than the non-HT group.

Recurrence
Sixteen studies were included, with a total sample size of 15,856 cases, and the RIS was 8,342 cases. TSA results demonstrated that the cumulative Z curve crossed the traditional boundary line, intersected the TSA boundary line, and reached the RIS line, indicating that the expected sample size had been reached, and the result was true positive, further verifying that the HT group had a lower risk of recurrence than the non-HT group.

Multifocality
Concerning multifocality, 44 articles with 34,235 cases were included. The RIS was 20,849 cases. The TSA results illustrated that the cumulative Z-curve crossed the traditional threshold line, intersected with the TSA threshold line, and reached the RIS line, indicating that the expected sample size had been reached. The result was positive, further verifying that the HT group had a higher multifocality risk than the non-HT group.

Vascular invasion
Seventeen studies were included for vascular invasion, with 14,105 cases sample size, and the RIS was 24,373 cases.The TSA results showed that the cumulative Z-curve crossed the traditional threshold line and intersected with the TSA threshold line, but did not reach the RIS line, indicating that although the expected sample size was not reached, positive results were obtained in advance, further verifying that the risk of vascular invasion in the HT group was lower than that in the non-HT group.

Bilaterality
Eighteen studies were included to evaluate bilaterality, with a total sample size of 12783 cases, and the RIS was 42465 cases. TSA results showed that the cumulative Z-curve did not reach the TSA threshold line and RIS line, indicating that the expected sample size was not reached. In the future, required to validate the reliability of the conclusion that the risk of bilaterality is higher in the HT group than that in the non-HT group.

Discussion
No consensus exists on the association between PTC and HT. To resolve this controversy, this study was performed to evaluate the relationship between the two conditions using a metaanalysis. Our analysis revealed that HT was associated with improvements in the clinicopathological characteristics and better prognosis of patients with PTC with lower risk of extrathyroidal extension, lower risk of distant metastasis, lower risk of lymph node metastasis, lower risk of vascular invasion, lower risk of recurrence rate, and a higher 20-year survival rate. Multifocal and bilaterality were positively correlated with HT. Since multifocal and bilaterality are thought to be features associated with PTC development, rather than with its deterioration, these findings are consistent with previous reports of a positive association between HT and PTC development and a protective effect of HT on PTC development [48]. Besides, PTC with HT had a risk of perineural infiltration.
There have been a number of proposed hypotheses to explain the linkage between HT and PTC. From a histological perspective, Tamimi et al. [77] assessed the prevalence and severity of thyroiditis among three types of surgically resected thyroid tumors and found a significantly higher rate of lymphocytic infiltration in patients with PTC. Nevertheless, PTC with concurrent HT is associated with less aggressive disease, less frequent capsular invasion, and less nodal metastasis [22]. Our result supported the result that HT may decrease the risk of lymph node metastasis and vascular invasion in patients with PTC. Similarly, Yoon et al. [70] and Donangelo et al. [78] reported that PTC with HT was significantly associated with a lower incidence of lymph node metastasis.
Furthermore, our findings showed that PTC patients with HT were also less likely to develop recurrence and have a higher 20-year survival rate, which were in agreement with prior studies [41,66]. Although we did not find the presence of HT indicates lower diseasespecific deaths, a recent study by Hu et al. reported that patients with HT had lower rates of tumor recurrence, and lower disease-related mortality compared with patients without HT [79]. Kashima et al. [13] reported a 0.7% cancer specific mortality and a 95% relapse-free 10-year survival rate in patients with HT compared to a 5% mortality and 85% relapse-free 10-year survival rate without chronic thyroiditis. The lymphocytic infiltration of HT may be an immunological response with a cancer-retarding effect, contributing to a favorable outcome of PTC versus other thyroid cancers [80].
Hypotheses about the mechanism of a better prognosis in PTC patients with HT have been evaluated in different ways [17]. HT is a kind of autoimmune disease that leads to the destruction of thyroid follicles through an immune response to a thyroid specific antigen. As PTC cells originating from the follicular cells would express the thyroid specific antigen, auto-antibodies from coexisting HT might destroy the tumor cells in much the same way as in HT alone [81]. Additionally, the infiltrated lymphocytes in patients with PTC are likely to be cytotoxic T cells acting as carcinoma cell killers, secreting interleukin-1 that inhibits thyroid cancer cell growth [82]. In a study on BRAF V600E , Xing et al. reported a significantly lower prevalence of BRAF V600E mutation in patients with PTC and HT, suggesting that HT is less likely to be associated with poor prognostic outcomes [83].
Interestingly, we observed that PTC patients with HT were younge than PTC patients without HT. We found that the results among age-balanced were similar to our original outcomes. Nevertheless, in the age-imbalanced groups, there were no differences in lateral lymph node metastasis, extrathyroidal extension, extrathyroidal extension, recurrence, multifocality, and bilaterality between PTC patients with HT and PTC alone. A study by Lun et al. also demonstrated that patients with PTC and HT were younger [56]. Zhang et al. reported older age is a risk factors for BRAF mutation in PTC patients, especially in those without HT [84]. This result suggests that age may be one of the potential sources of bias. More studies are needed in the future with a larger sample size and rigorous design to confirm our findings.
The strengths of the current study need to be mentioned. This was an updated meta-analysis including more studies and more outcomes. There was no apparent publication bias, leading to the research results being more reliable and convincing. Besides, we used TSA to further validate our findings. However, residual confounding variables were a problem. Uncontrolled or unmeasured confounding factors have the potential for bias, and the possibility that residual confounders influenced the results cannot be ruled out. Our analysis was largely limited by the retrospective nature of most of the included studies where clinical details were usually not available. More prospective studies with longer follow-ups are needed to further elucidate this relationship.

Conclusions
This meta-analysis shows a clinical relationship between two disease entities. PTC patients with HT may have lower incidence of extrathyroidal extension, distant metastasis, lymph node metastasis, vascular invasion, and better prognosis than patients with PTC alone.